Recent developments in integrated circuitry, enabling the economical manufacture of filters, coding/decoding (codec) circuits, line interfaces and the like, have facilitated the conversion of outgoing voice samples from analog to digital form and the reconversion of incoming digitized samples to analog form at the subscriber end of a telephone line; this, in turn, allows the use of such a line for both voice communication and data transmission. Reference in this connection may be made to a report published October 1980 by the European Communication Commission (CEPT), titled "First Report of CEPT Special Group on ISDN (GSI)", with ISDN standing for "Integrated-Services Digital Network".
Thus, a subscriber line of such a network may operate at a rate of 80 kbit/sec accommodating a 64-kbit/sec voice channel and a so-called .DELTA. channel available for data transmission and signalization. More particularly, a time slot of 125 .mu.s can include a voice byte, a data bit and a synchronizing bit. This corresponds to a rate of 8 kbits/sec for data meassages, though a possible doubling of that bit rate is also contemplated.
The .DELTA. channel, carrying the signalization, must be accessible to all terminals whereas the voice channel is to be used only by a telephone station or possibly by a terminal designed for high-rate data transmission. The remaining terminals can use the .DELTA. channel for the transmission of data at the aforementioned low rate. The signalization present on the latter channel, incidentally, also serves for the assignment of the voice channel to a particular subscriber.
Whereas data transmission on the voice channel is also carried out in the form of 8-bit words or bytes, data carried on the .DELTA. channel are generally grouped into messages or packets in conformity with a high-level protocol of HDLC type. See, in this connection, a 1976 draft by the International Organization for Standardization (ISO/DIS 4335) titled "Data communication--High-level data-link control procedures--Elements of procedures". With the particular protocol there envisaged, the release of a channel previously seized for data transmission is indicated by an unbroken succession of at least seven bits of logical value "1". Reference may also be made to an article titled "Packet Switching in Radio Channels" by Leonard Kleinrock and Fouad A. Tobagi, IEEE Transactions on Communications, Vol. COM-23, No. 12, December 1975, describing a carrier-sensing multiple-access system.
In a system of the type referred to above, with a number of data terminals given random access to a .DELTA. channel of a telephone line, it is necessary to provide means for preventing a collision between messages from two or more terminals concurrently ready for transmission. Even when the channel is found to be idle, several terminals may simultaneously seek access, thereby creating a conflict.
In the so-called Aloha system, described by Norman Abramson and Franklin F. Kuo in Computer-Communication Networks published 1973 by Prentice-Hall, Inc., the problem of possible interference between concurrently transmitted messages is solved by blocking the transmission of any overlapping packets and thereafter attempting their retransmission at different times. As the likelihood of collision increases with the number of potentially competing terminals, the channel may remain unused for extended periods in a system of this type.